1. Field of the Invention
The present invention relates to an apparatus and a method for forming an intermediate original sheet (a transparent sheet with original films arranged fixed on it being in a desired layout, to be utilized for manufacturing a printing plate by a photographic method) required in intermediate steps of a photomechanical process.
2. Description of the Background Art
In a process of making a book, each of a number of printing sheets typically includes a plurality of pages printed in a prescribed layout. Each printing sheet is folded to be what is known as a signature. A plurality of signatures are gathered for binding. A bound book is trimmed for size adjustment and thus the respective pages of the signatures are separated from each other.
It is necessary to arrange and print the plural pages on a printing sheet so that they are lined up each in a correct order and in a correct direction after the folding. In a process of printing a book, an original film corresponding to each page has an image of a common standardized size such as A4 or A3. In a prepress process of forming a printing sheet, a plurality of original films of a standard size are stripped on a base sheet. The layout of the original films should take into account the later formation of the signature. In the following, procedures for stripping original films on a base sheet will be described. The base sheet on which the original films are stripped is referred to hereinafter as an "intermediate original sheet."
(1) Determination of a Layout of the Original Films
Referring to FIG. 1A, in the case of stripping four two sided pages, individual pages 1, 4, 5 and 8 of a book for example are printed on one surface of a printing sheet. Referring to FIG. 1B, pages 2, 3, 6 and 7 of the book are printed on the reverse surface of the printing sheet.
Referring to FIGS. 1A and 1B, note that page 2 is located on the reverse of the page 1, page 3 is located on the reverse of the page 4, page 6 is located on the reverse of the page 5, page 7 is located on the reverse of the page 8. The pages 3, 4, 5 and 6 are printed upside down with respect to pages 2, 1, 8 and 7, respectively.
Referring to FIG. 1C, when the above-mentioned printing sheet is folded, the respective pages are lined up in the correct order in the correct direction. The number of pages on the printing sheet, or the order of folding differs dependent on the dimensions of the printing machines or the kind of binding. Accordingly, the layout of the pages printed on the printing sheet differs dependent on those restrictions. The operator who strips the pages determines the arrangement of the original films of the respective pages on the base sheet based on the layout of the page. The determination of the arrangement includes positioning and angling of the original film of each page to be stripped on the base sheet.
(2) Cutting of Unnecessary Portions of the Original Films
Generally, an image of a page is formed on a central area of an original film. Accordingly, there exist portions without any image formed thereon around the image. The original film normally has a rectangular form larger than the size of the image. Register marks for positioning are printed on the portions of the original film where the image is not formed (unnecessary portions). The unnecessary portions of each original film should not be stripped in being overlapped with other original films. In order to avoid such overlapping, the unnecessary portions are cut in advance.
A margin of several millimeters is sometimes set between the adjacent images of the pages on the base sheet. This margin is called a gutter. Referring to FIG. 2, four original films OF1 to OF4 are stripped on a base sheet BS. A spacing Xl is provided as a gutter between the images of the original films OF1 and OF2. A gutter of the same spacing is also provided between the images of the original films OF3 and OF4. A gutter of a spacing Yl is provided between the images of the original films OF1 and OF3, and between the images of the original films OF2 and OF4.
In the above described layout, portions to overlap with the adjacent original films out of the unnecessary portions of the respective original films OF1 to OF4 are cut manually so that the above-mentioned gutters are formed. Referring to FIG. 3, for example in the case of the original film OF1, the hatched portions are cut out of the unnecessary portions of the original film OF1.
If no gutter is set, Xl and Yl are both zero. In other words, the portions adjacent to the respective other original films out of the unnecessary portions of the films are entirely cut.
(3) Registration and Stripping of Original Films on a Base Sheet
Referring to FIG. 3, register marks RM1 to RM4 for registration are formed on peripheral portions of the original film. Reference marks to be coincident with the register marks RM1 to RM4 are provided on a transparent base sheet BS (such as those shown in FIGS. 2 and 4). The reference marks are sometimes given on layout paper on which the transparent base sheet is placed.
The operator arranges the respective original films on the base sheet according to the above-mentioned layout by visually confirming coincidence between the register marks RM1 to RM4 and the reference marks on the base sheet. The original films thus arranged are fixed on the base sheet by an adhesive tape or the like.
As a result of the operations of (1) to (3), the stripping of the original films on the base sheet for forming a printing sheet is completed. In the case of monochromatic printing, a plate for the monochromatic printing may be prepared from the base sheet thus completed.
In the case of color printing, it is necessary to effect the stripping on the base sheet for each color separation (normally, for each of yellow (Y), magenta (M), cyan (C) and black (Bk)). Stripping of original films by manual operation as described above takes time and labor. In practice, original films are typically stripped for each color according to the following procedures. The following method and the apparatus used for that method are described for example in U.S. patent application Ser. No. 20334 filed by Nishida et al, entitled "An apparatus for Composing a Plurality of Original Films on a Base Sheet".
Original films for any one of the four colors Y, M, C and Bk are manually stripped on a base sheet, so that a master sheet is formed. The arrangement of the original films on the master sheet is read in by a computer as numerical data, utilizing a digitizer or the like. The original films for the remaining three colors are stripped on base sheets for the respective color separations based on the read numerical data. Since the apparatus for automatically stripping the original films on the base sheet based on the numerical data is disclosed in the above mentioned application, a detailed description thereof is not repeated here.
The master sheet and the three base sheets on which the original films of the respective colors are stripped based on the numerical data read from the master sheet constitute a complete set of printing sheets for the four color separations for color printing.
The reading of the numerical data from the master sheet by using a digitizer or the like usually includes some error. There is a small deviation between the arrangement of the original films on the master sheet and the arrangements on the other three base sheets, due to input error by the digitizer. If color printing is effected based on those base sheets, defective printing may occur due to color deviation. In many cases, the original films on the master sheet are temporarily removed and stripped again on a base sheet by the apparatus for automatically stripping the original films on the base sheet in the same manner as in the other three colors. Thus, the arrangements of the original films on the base sheets of the four colors coincide with each other.
However, it is pointed out that the conventional techniques involve disadvantages as described below at the time of stripping original films on base sheets.
One of the disadvantages is that in the case of manual stripping on a master sheet, the stripping accuracy cannot be maintained at a preferable level. As shown in FIG. 3, if two register marks RM3 and RM4 out of the four register marks are cut off, registration is effected only by using the remaining register marks RM1 and RM2. In such a case, the accuracy of registration is lowered as a matter of course compared with the case of using the four register marks including the marks RM3 and RM4. The quality of a printing sheet is lowered due to color deviation.
FIG. 4 shows the most unfavorable case. If original films of eight pages are stripped on a base sheet BS, each of the original films OF2, OF3, OF6 and OF7 arranged at the center has three sides to be cut, adjacent to other original films. A register mark is left only on the remaining side of each original film. It is practically impossible to position each original film with accuracy by using only one register mark. Therefore, the accuracy of registration is considerably lowered.
Another disadvantage is that work for preparing original films to be stripped takes much time. Although the operation for stripping original films is carried out automatically, operation for cutting original films according to a layout is carried out manually. Since heads and tails of the respective pages, i.e., are not constantly fixed, it is necessary to confirm the portions to be cut for each page by taking account of the layout at the time of cutting the original films. This operation not only fatigues the operator but also takes much time.
A further disadvantage is that an inexperienced operator cannot strip original films with precision according to the conventional stripping techniques. Stripping on a master sheet needs to be carried out manually. An inexperienced operator might prepare a master sheet with inaccuracy. Since stripping on a base sheet for each of the other three colors is effected based on the master sheet, the quality of a printed matter obtained would be lowered.